Position detection apparatus with adjustable beam and interference fringe positions

ABSTRACT

A position detection apparatus has a substrate on which a diffraction grating is formed and an alignment optical system for illuminating the diffraction grating with a pair of coherent light beams having different frequencies from each other from different directions. The intensity of interference fringes formed due to the interference of diffracted beams generated in the diffraction grating is detected photo-electrically. The alignment optical system forms the pair of coherent light by using an optical modulator, and two luminous fluxes from the optical modulator pass through independent optical paths positioned symmetrically with the optical axis of the alignment optical system therebetween and reach the diffraction grating from different directions. The alignment optical system has a stop having an opening having an inclined edge with respect to the direction of the grating components of the diffraction grating, the opening being in conjugation with the substrate.

This is a continuation of application Ser. No. 482,557 filed Feb. 21,1990, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heterodyne coherent type positiondetection apparatus, and, more particularly, to an apparatus suitablyused for accurate alignment in an apparatus for manufacturingsemiconductors.

2. Related Background Art

In recent years, a projection type exposing apparatus, so-called astepper has been widely used as an apparatus for transcribing finepatterns on the surface of a semiconductor devices or the like onto thewafer of a semiconductor. In particular, since high density mount LSIsmanufactured by the above-described technology have been desiredrecently, finer pattern must be transcribed to wafers. In order toachieve this, more accurate alignment is necessary.

An accurate position detection apparatus in accordance with theheterodyne coherent method is known, as disclosed in, for example,Japanese Patent Laid-Open No. 62-261003 and U.S. Pat. No. 4,710,026.

The former apparatus is arranged in such a manner that its aligninglight source comprises a Zeeman laser utilizing a Zeeman effect whichemits luminous flux including p-polarized light and s-polarized lighthaving slightly different frequencies. The luminous flux emitted fromthe Zeeman laser is, by a polarizing beam splitter, divided into twofluxes: p-polarized light having a frequency of f₁ and s-polarized lighthaving a frequency of f₂. The thus formed two fluxes irradiate adiffraction grating mark formed on a reticle (or a mask) frompredetermined two directions via two corresponding reflecting mirrors.Since the mask has an opening neighboring the diffraction grating mark,a portion of each of the two fluxes can, from predetermined twodirections, irradiate the diffraction grating mark formed on the waferafter passing through the opening.

As a result, the diffraction grating mark on the mask and that on thewafer respectively generate diffracted beams. The diffracted beams(p-polarized light and s-polarized light) emitted from the mask markinterfere with each other via a polarizing plate. As a result, a singleoptical beat signal is generated. Also the diffracted beams from thewafer mark interfere with each other via the polarizing plate. As aresult, another optical beat signal is generated. The thus generated twooptical beat signal are photoelectrically detected so that the relativephase difference between the two signals is detected. Since the phasedifference corresponds to the quantity of deviation between the twoluminous fluxes intersecting on the diffraction grating mark and thesubstrate, an accurate alignment can be achieved by relatively movingthe reticle and the wafer in such a manner that the phase differencebecomes zero.

However, the above described conventional apparatuses arise a problem inthat an error takes place due to the following factors:

The first factor lies in that the perfect splitting of polarized beamscannot be achieved if a slight positional deviation from the designedpositions takes place between the active plane of a polarization beamsplitter of a polarization splitting device such as the above-describedpolarization beam splitter for splitting two polarized beams from theZeeman light source and the polarizing plane on which the two polarizedbeams are made incident. As a result, the split luminous flux includesnot only the main polarized beam component but also a small quantity ofthe other polarized beam component, that causes noise.

The above-described positional deviation is caused by errors that havemainly taken place at the time of manufacturing the semiconductors.However, the polarization splitting device such as the polarization beamsplitter cannot perfectly split p-polarized light and s-polarized light.Therefore, even if the optical system comprising the above-describeddevices can be mounted without the manufacturing errors, each ofpolarized beams which has been split includes a small quantity of noisewhich is different in the polarization component and in the frequencyfrom the main polarized beam component. The split polarized beams becomecoherent each other so that an optical beat is generated between themain polarized beam component and the noise component, causing anexcessive error to appear in the optical beat signal which has beenphotoelectrically detected.

A second factor is caused by the fact that linearly polarized luminousflux is, strictly, deformed into elliptically polarized light due to thepassage of the flux through optical devices such as a polarization beamsplitter and a reflecting mirror. Usually, a laser beam emitted from alight source reaches the diffraction grating mark on the mask or on thereticle after it has been subjected to one or more reflections. Duringthe process of the passage of the laser beam, a noise factor of the typeshown in FIGS. 4 and 7 is generated That is, noise component Pr(frequency: f₁) is generated in direction x which is perpendicular tomain polarized beam component Pry (frequency: f₁) in direction y.Furthermore, noise component Ny (frequency: f₂) is generated indirection y which is perpendicular to noise component Nx (frequency: f₂)in direction x. Although the noise components (Prx and Nx) which aregenerated perpendicularly to the main polarizing direction can beeliminated, the noise component Ny in the same direction as mainpolarized beam component Pry cannot be eliminated. Therefore, beatsgenerate between the main polarized beam component Pry (frequency: f₁)and the noise component Ny (frequency: f₂) in the same direction as themain polarized beam component Pry, causing the optical beam signal whoseposition is to be detected to include an excessive error.

Now, the above-described problem will be analyzed with reference to thetwo diffracted beams to be photoelectrically detected.

The amplitude D₁ of the diffracted light consisting of the mainpolarized beam component having the frequency of f₁ and the noisecomponent having the frequency of f₂ included in the main polarized beamcomponent can be expressed as follows:

    D.sub.1 =U.sub.1 exp [-i(k.sub.1 t-φ.sub.1)]+V.sub.2 exp [-i(k.sub.2 t-φ.sub.2)]                                           (1)

On the other hand, the amplitude D₂ of the diffracted light consistingof the main polarized beam component having the frequency of f₂ and thenoise component having the frequency of f₁ included in the mainpolarized beam component can be expressed as follows:

    D.sub.2 =U.sub.2 exp [-i(k.sub.2 t+φ.sub.2)]+V.sub.1 exp [-i(k.sub.1 t+φ.sub.1)]                                           (2)

where U₁ and V₂ respectively represent the amplitude of the mainpolarized beam component (frequency: f₁) and that of the noise component(frequency: f₂) of the diffracted light having the amplitude D₁, U₂ andV₁ respectively represent the amplitude of the main polarized beamcomponent (frequency: f₂) and that of the noise component (frequency:f₁) of the diffracted light having the amplitude D₂, k₁ (=2πf₁) and k₂(=2πf₁ ) represent the wave numbers, and φ₁ and φ₂ represent the phasechanges generated when the diffracted beams which respectively have thefrequencies f₁ and f₂ are subjected to the diffraction by means of thediffraction grating.

Intensity of the optical beam signal which is photoelectrically detectedby a detector can be expressed as follows from thus obtained Equations(1) and (2): ##EQU1## Expansion of Equation (3) gives ##EQU2##

The optical beat signal obtained from Equation (4) originally includesonly the phase component φ₁ +φ₂ representing the positional informationof the diffraction grating. It is apparent that the optical beat signalfurther includes other phase components (φ₁ -φ₂) and (-φ₁ +φ₂). Assumingthat V₁ and V₂ are sufficiently small with respect to U₁ and U₂, thephrase error e can be expressed by the following equation: ##EQU3##

Assuming that the light intensity ratio of the polarized componentsperpendicular to each other and included in each of two luminous fluxessplit by the above-described polarization beam splitter is 1:1000 (|V₂|² :|U₁ |² or |U₂ |² : |V₁ |²) and U₁ =U₂ and V₁ =V₂, the phase error eaccording to Equation (5) becomes ##EQU4##

Therefore, the optical beat signal which is photoelectrically detectedinevitably includes an error which cannot be disregarded.

The conventional apparatus has been arranged in such a manner that thepitch of interference fringes generated with the mark on the diffractiongrating formed on the substrate is illuminated with two luminous fluxeshaving directions frequencies from two direction becomes half of thepitch of the diffraction gratings. Therefore, the phase differencebetween the optical beat signal including positional information of thediffraction grating mark and the reference optical beat signal changesby 2π whenever the relative deviation between the two luminous fluxesand the substrate becomes the half pitch. That is, the range which canbe detected as the phase difference becomes 2π.

Then, assuming that the pitch of the mark on the diffraction gratingformed on the substrate is P, the above-described phase error e can beconverted into the following relative deviation δ between the twoluminous fluxes and the substrate: ##EQU5##

Assuming that the pitch P of the mark on the diffraction grating is 10μm, the above-described phase error of 3.6° can be converted into thedeviation δ expressed by: ##EQU6## It is apparent that theabove-described error cannot be disregarded.

When, for example, a 4-megabit VLSI is manufactured, a printing linewidth which is about 0.6 to 0.7 μm is necessary. In order to achievethis, the detector must have an accuracy of at least 10% (which is 0.06to 0.07 μm) of the line width (about 0.6 to 0.7 μm). However, the errorobtained in accordance with Equation (8) causes a difficulty inachieving an alignment at the time of subjecting 4-megabit VLSI to anexposure.

The above-described conventional apparatus has experienced anotherdetection accuracy problem due to diffracted light from a field stop. Inorder to overcome the problem of this type, it is preferable that thefield stop be positioned in conjugation with the position of a wafer inan optical system for transmitting alignment light when theabove-described heterodyne type position detection is conducted. Thus,the illumination region of the mark on the diffraction grating of thewafer may be restricted. The reason for this is that the detector shouldbe protected from reflected light acting as noise and generated when aportion of alignment light is applied to the pattern in thetranscription region or another alignment mark.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a position detectionapparatus capable of obtaining a reliable and stable optical beatsignal.

Another object of the present invention is to provide a positiondetection apparatus capable of accurately detecting the position.

The apparatus according to present invention has an alignment opticalsystem for applying two luminous fluxes having different frequenciesfrom each other from two directions to a diffraction grating formed on asubstrate so as to be position-detected. Diffracted light beamsgenerated from the two luminous fluxes are made to interfere with eachother and the optical beat signal generated due to the interference. Thealignment optical system comprises a light source for supplying luminousfluxes for illuminating the diffraction grating, a beam splitter forsplitting the illuminating luminous fluxes into two portions, an opticalmodulator for modulating the frequency of each of the luminous fluxeswhich have been split into two portions by a predetermined value, andmeans for adjusting the optical path for the two luminous fluxes in sucha manner that the split two luminous fluxes pass with the optical axisof the alignment optical system positioned therebetween.

As an aspect of the present invention, the optical path adjustment meanshas means for adjusting the intersection angle of the two luminousfluxes for illuminating the diffraction grating formed on the substrateand means for adjusting the direction of generation of the interferencefringes.

According to the above-described position detection apparatus, the twoluminous fluxes which have been modulated so as to have differentfrequencies always pass through different optical paths. Therefore, thenoise component which degraded the detection signal can be eliminated.Therefore, a stable optical beat signal having high contrast can beobtained, causing an accurate alignment to be conducted. Furthermore,the optical paths for the two luminous fluxes can be adjusted, and theintersection angle of the two luminous fluxes for illuminating thediffraction grating mark formed on the substrate can be adjusted. Inaddition, the rotational deviation between the direction of thearrangement of the interference fringes and the direction of thearrangement (pitch) of the diffraction grating mark can be adjusted.

A further object of the present invention is to provide a positiondetection apparatus having a field stop which does not influence thedetection accuracy at a conjugation position with the diffractiongrating mark in the alignment optical system.

The apparatus according to the present invention has an alignmentoptical system for supplying two luminous fluxes for illuminating thediffraction grating mark from different directions, and stop means ispositioned in conjugation with the diffraction grating mark of thealignment optical system so that the opening portion of the stop meansis appropriately formed. The edge is formed in the longitudinaldirection of the grating component of the diffraction grating.

It is preferable that the pair of confronting edges meet the followingrelationship assuming that the length of the edge forming the openingportion of the stop means is L, the pitch of the interference fringesformed on the stop is P_(FS) and an integer is represented by m:

    L=mP.sub.FS.

It is preferable that the opening of the stop be formed in aparallelogram. Specifically, it is preferable that the followingrelationship be held: ##EQU7## where the inclination of said edge ofsaid opening is θ, the pitch of said interference fringes formed on saidstop means is P_(FS), the pitch of said diffraction grating is P_(WM),the length of said edge of said opening of said stop means extending insaid direction corresponding to said direction of said arrangement ofsaid diffraction grating is W, the length of said edge of said openingof said stop means extending in a direction corresponding to a groove ofsaid diffraction grating is h and allowable detected error on saidsubstrate is X_(WM).

A light shield means may be formed at a position closer to thediffraction grating than the position of the stop means for the purposeof shielding diffracted light from the stop.

It is further preferable that the following relationship be held.##EQU8##

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the schematic structure of a first embodiment of aprojection exposing apparatus according to the present invention;

FIG. 2A is a plan view which illustrates a diffraction grating mark RMon a reticle;

FIG. 2B is a plan view which illustrates a diffraction grating mark WMon a wafer;

FIGS. 2C and 2D are plan views which respectively illustrate a fieldstop;

FIG. 3 is a perspective view which illustrates the alignment opticalsystem shown in FIG. 1;

FIG. 4 illustrates a state in which the main polarized component and thenoise component having the same frequency are elliptically polarized;

FIG. 5 is a perspective view which schematically illustrates the opticalpath adjustment function;

FIG. 6A illustrates a state in which the interference fringe and thediffraction grating mark deviate considerably from each other;

FIG. 6B illustrates a state in which the direction of the interferencefringe and that of the diffraction grating mark are compensated and madeto coincide with each other;

FIG. 6C illustrates a state in which the interference fringe and thediffraction grating mark, which had been deviated from each other by aslight degree, have been compensated;

FIG. 7 illustrates a state in which the main polarized beam component ofthe noise component having a different frequency areelliptically-polarized;

FIG. 8 illustrates the schematic structure of a second embodiment of aprojection exposing apparatus according to the present invention;

FIG. 9 illustrates the optical path of the alignment optical systemshown in FIG. 8;

FIG. 10 illustrates the intensity distribution of diffracted light;

FIG. 11 illustrates generation of diffracted light by the diffractiongrating mark of the wafer;

FIG. 12 illustrates the phase error place in the optical beat signal tobe detected;

FIG. 13 illustrates the amplitude distribution of diffracted light;

FIG. 14 is a plan view which illustrates the FS (Field Stop) shown inFIG. 8;

FIG. 15 illustrates the secondary intensity distribution of alignmentbeams LB₁ and LB₂ on a pupil surface of a projecting lens;

FIG. 16 illustrates the secondary intensity distribution of alignmentbeam LB₁ on a pupil surface of a projecting lens;

FIG. 17 illustrates the optical path showing the total magnifications oflenses;

FIG. 18 is a graph which illustrates the values of Ab;

FIG. 19 is a plan view which illustrates the FS (Field Stop) accordingto a third embodiment;

FIG. 20 is a graph which illustrates the values of Ab obtained inaccordance with the third embodiment;

FIG. 21 is a schematic structure of a fourth embodiment of a projectionexposing apparatus according to the present invention;

FIG. 22A is a plan view which illustrates the shape of a reticle markRM;

FIG. 22B is a plan view which illustrates the shape of a wafer mark WM;

FIGS. 23A and 23B are plan views which respectively illustrate a maskmember; and

FIG. 24 illustrates generation of diffracted light on the diffractiongrating mark of the wafer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the schematic structure of a first embodiment of aposition detection apparatus according to the present invention.

A reticle (a mask) 1 having a predetermined circuit pattern and analignment diffraction grating mark RM is held by a reticle stage 2 whichcan be moved two dimensionally. Patterns on the reticle 1 are imaged ona wafer 4 (a substrate) with exposing light emitted from an illuminatingoptical system 40 by a projecting objective lens 3. Also the wafer has adiffraction grating mark WM similar to the diffraction grating mark RMformed on the reticle 1.

The wafer is absorbed onto the stage 5 which is moved two-dimensionallyin accordance with a step-and-repeat method until the transcription ofthe pattern on the reticle 1 in a single shot region on the wafer iscompleted. Then, the wafer is stepped to the next shot position. Anindependent interferometer (omitted from illustration) is provided foreach of the stages so that x-directional, y-directional and rotational(θ) positions on the reticle stage 2 and the wafer stage 5 are detected.The stages are moved in each of the predetermined directions by anoperating motor (omitted from illustration).

Exposing light supplied from an exposing illumination optical system 40is reflected downwards by a dichroic mirror 37 or the like diagonallypositioned above the reticle 1 by 45° so that the reticle 1 is uniformlyilluminated. The pattern on the reticle 1 which is being illuminateduniformly is imaged on the wafer by the projecting lens 3.

An alignment optical system 10 to 31 for detecting position is providedabove the dichroic mirror 37.

Now, the alignment optical system will be described.

Aligning illumination light having a wavelength different from that ofthe above-described exposing light becomes circularly-polarized light bya quarter wavelength plate 11 after it has been emitted from a laserlight source 10. A luminous flux is divided into p-polarized light ands-polarized light having the same light quantity by a polarization beamsplitter 13 after it has passed through a lens 12.

The s-polarized luminous flux reflected by the polarization beamsplitter 13 is made incident upon a first acoustooptic modulator 15a (tobe called simply "an AOM 15a" hereinafter). On the other hand,p-polarized luminous flux which has passed through the polarization beamsplitter 13 is made incident upon a second acoustooptic modulator 15b(to be called simply "an AOM 15b" hereinafter) via a reflecting mirror14. The AOMs respectively modulate the frequency of the p-polarizedluminous flux into f₁ and that of the s-polarized luminous flux into f₂so as to cause the frequency difference between the p-polarized luminousflux and the s-polarized luminous flux to be made Δf. The s-polarizedluminous flux whose frequency has been modulated into fl by the AOM 15areaches a polarization beam splitter 18 via a parallel plane plate 16aand a reflecting mirror 17. On the other hand, the p-polarized luminousflux whose frequency has been modulated into f₂ reaches the beamsplitter 18 via a parallel plane plate 16b.

The parallel plane plates 16a and 16b serve as optical path adjustmentmeans and are inclined with respect to a direction are the two luminousfluxes whose frequencies have been modulated into different frequenciesso that the two luminous fluxes are not again synthesized by the beamsplitter 18, the degree of inclination being arranged to be adjustable.As a result, the optical paths for the two luminous fluxes whosefrequencies have been modulated by the corresponding AOMs are shifted bya predetermined quantity so that the two luminous fluxes pass throughthe polarization beam splitter 18 separated from each other. Thepolarization beam splitter 18 is disposed in the vicinity of the pupilin the alignment optical system. The functions of the parallel planeplates 16a and 16b will be described later.

The polarizing directions of the two separated luminous fluxes runningin parallel to each other are rotated by 45° by a half-wave plate 19.After the two luminous fluxes have passed through a polarization beamsplitter 20, the p-polarized light component of each of the luminousfluxes passes in the transmitting direction, and the s-polarized lightcomponent of the same passes in the reflecting direction.

The two luminous fluxes which have passed through the polarization beamsplitter 20 are converged onto a reference diffraction grating 22 by alens 21 so that interference fringe moving in the direction of the pitchare formed. Diffracted light which has passed through the diffractiongrating 22 is photo-electrically detected by a detector 23 as areference optical beat signal.

The two luminous fluxes which have been reflected by the polarizationbeam splitter 20 reach a beam splitter 26 disposed in the vicinity ofthe pupil of the alignment optical system via relay system 24a, 24b and25. The two luminous fluxes running parallel to each other and whichhave passed through the beam splitter 26 pass through a parallel planeplate 35 whose inclination angle can be varied with respect to theoptical axis of the alignment optical system for the purpose ofmaintaining a telecentric relationship. Then, the two luminous fluxesilluminate the diffraction grating mark RM on the reticle 1 in twodirections which make a predetermined intersection angle via objectivelens 36 and a dichroic mirror 37. The parallel plane plate 35 providedfor the purpose of maintaining the telecentric relationship is disposedin the pupil space of the alignment optical system, preferably disposedin the vicinity of the position of the pupil.

The parallel plane plate 35 may be structured by combining a parallelplane plate having a large thickness for the purpose of conducting acoarse adjustment and a parallel plane plate having a small thicknessfor the purpose of conducting fine adjustment.

In the case where the projecting lens 3 does not have chromaticaberration with respect to alignment light, it is preferable that theobjective lens 36 be composed by a bi-focal point optical systemdisclosed by one of the inventors of the present invention in JapanesePatent Laid-Open No. 63-283129 (U.S. Ser. No. 192,784 filed on May 10,1988), which was abandoned in favor of continuation application Ser. No.469,713 filed Jan. 24, 1990, which was abandoned in favor ofcontinuation application Ser. No. 536,939 filed June 12, 1990, which isnow U.S. Pat. No. 5,004,348 issued Apr. 2, 1991. In this case, in orderto divide the two polarized luminous fluxes made incident upon thebi-focal point optical system, the structure is arranged in such amanner that the polarization direction of the incident fluxes inclineswith respect to the optical axis of the bi-focal point optical system.As a result, the luminous fluxes passing toward the first focal pointare imaged on the reticle, while the other luminous fluxes passingtoward the second focal point are imaged at a position which is outsidethe reticle, and they are imaged on the wafer via the projecting lens 3.

As shown in FIG. 2A, the reticle 1 has an opening P0 through whichaligning light passes through, the opening P0 formed in parallel to thediffraction grating mark RM. As shown in FIG. 2B, a diffraction mark WMhaving the same pitch as that of the diffraction grating mark formed onthe reticle is formed on the wafer at a position corresponding to theabove-described opening P0. As a result, a portion of alignment lightilluminates the diffraction grating mark RM on the reticle from twodirections making a predetermined intersection angle. As a result,interference fringes are generated moving in the direction of the pitchof the diffraction grating. The intersection angle of the two luminousfluxes has been predetermined so as to make ±1-order diffractionreflected light generated from the diffraction grating mark RM pass inthe direction of the optical axis.

As a result, the ±1-order diffraction reflected light generated from thediffraction grating mark RM again passes through the dichroic mirror 37and the objective lens 36, and then it reaches a field stop 32 via theparallel plane plate 35, the beam splitter 26, the lens 27 and the beamsplitter 28.

The field stop 32 is positioned in conjugation with the position of thereticle 1. As shown in FIG. 2C, an opening portion S_(RM) is formed at aposition corresponding to the diffraction grating mark RM in order topass only diffracted light from the diffraction grating mark RM on thereticle 1. The diffracted light from the diffraction grating mark RM issubjected to a filtering by a spatial filter 33 for cutting 0-orderlight (positive reflected light) after it has passed through the fieldstop 32 so that an optical beat signal including positional informationof the reticle 1 is photo-electrically detected by a detector 34.

On the other hand, a portion of the aligning luminous flux which haspassed through the opening P0 formed in the reticle illuminates thediffraction grating mark WM on the wafer in two directions making apredetermined intersection angle via the projecting lens 3. As a result,±1-order diffracted light generated at the mark WM and passing along theoptical axis reaches a field stop 29 via the projecting lens 3, thedichroic mirror 37, the objective lens 36, the parallel plane plate 35,the beam splitter 26, the lens 27 and the beam splitter 28.

The field stop 29 is positioned in conjugation with the position of thewafer 4. As shown in FIG. 2D, an opening portion S_(WM) is formed at aposition corresponding to the diffraction grating mark WM in order topass only diffracted light from the diffraction grating mark WM on thewafer 4. The diffracted light from the diffraction grating mark WM issubjected to a filtering by a spatial filter 30 for cutting 0-orderlight (positive reflected light) after it has passed through the fieldstop 29 so that an optical beat signal including positional informationof the wafer 4 is photo-electrically detected by a detector 31.

The spatial filters 30 and 33 are positioned in substantial conjugationwith the pupil surface of the alignment optical system, that is,positioned in substantial conjugation with the pupil (the exit pupil) ofthe projecting lens so that 0-order light (positive reflected light)from the diffraction grating marks formed on the reticle and the waferis stopped and only ±1-order diffracted light (diffracted lightgenerated in the perpendicular direction with respect to the diffractiongratings of the reticle 1 and the wafer 4) is allowed to pass through.The detectors 31 and 34 are respectively positioned in substantialconjugation with the reticle 1 and the wafer 4 via the objective lens 36and the lens 27.

When the reticle and the wafer are stopped at optional positions in astate where the position has not been aligned yet, all of the threephotoelectric signals obtained from the detectors 23, 31 and 34 becomesine wave optical beat signals each having a frequency of Δf, the threephotoelectric signals being deviated from one another by a predeterminedphase difference. The phase difference (±180°) among the optical beatsignals from the reticle 1 and the wafer 4 corresponds to the relativepositional deviation which is within half of the grating pitch of thediffraction gratings formed on the reticle 1 and the wafer 4.

When the reticle and the wafer have been moved relatively to each otherin the direction in which the gratings are arranged, optical beatsignals of the same phase are generated whenever the relative positionaldeviation becomes half of the grating pitch of the diffraction gratingmarks RM and WM. Therefore, prealignment with an accuracy better than1/2 grating pitch or less of the diffraction grating marks RM and WM canbe conducted. A main control system 51 two-dimensionally moves thereticle stage 2 or the wafer stage 5 by using its servo system 52 insuch a manner that the phase difference obtained by a phase differencedetection system 50 becomes zero. As a result, high resolution positiondetection can be achieved.

Another arrangement may be employed in which the reference optical beatsignal obtained by the detector 23 is arranged to be a reference signaland the position alignment is conducted in such a manner that the phasedifferences among the above-described reference signal, and the opticalbeat signals from the diffraction grating marks RM and WM become zero.Furthermore, drive signals for driving the AOMs 15a and 15b may, ofcourse, be used as reference signals.

Now, this embodiment arranged in such a manner that the optical beatsignal, which has been photoelectrically detected, does not include anyerror will be described.

As shown in FIG. 3, the luminous flux supplied from the alignment lightsource 10 becomes circularly polarized light by the quarter wavelengthplate 11 and is separated into s-polarized light and p-polarized lightwhen it passes through the polarization beam splitter 13. Assuming thatnoise components Δp and Δs polarized perpendicularly to the polarizationdirection of the s-polarized light and the p-polarized light areincluded in the s-polarized light and the p-polarized light by smallquantities, each of the polarization components of the two luminousfluxes which have respectively passed through the AOMs 15a and 15b issubjected to the same frequency modulation. Then, the two luminousfluxes S_(f1) +Δp_(f1) and P_(f2) +Δs_(f2) pass through the parallelplane plates 16a and reflecting mirror 17, pass through parallel planeplate 16B, respectively, and reach the polarization beam splitter 18.The polarization beam splitter 18 guides each of the main polarized beamcomponents in predetermined directions and guides the major portion ofthe noise components Δp_(f1) and Δs_(f2) in another direction.Therefore, the two luminous fluxes which have passed through thepolarization beam splitter 18 are brought to a state in which only asubstantially pure main polarized beam component exists.

The polarization direction of each of luminous fluxes S_(f1) ' andP_(f2) ' which have passed through the polarization beam splitter 18 isrotated by 45° about the optical axis after they have passed through thehalf-wave plate 19 so as to reach the polarization beam splitter 20 insuch a manner that the main polarization directions cross at rightangles. The polarization beam splitter 20 strictly splits the mainpolarized beam component S_(f1) ' having the frequency f₁ into P_(f1) "and S_(f1) " in corresponding directions x and y. Furthermore, Δs_(f1) "and Δp_(f1) " exist in a mixed manner as noise components each having afrequency f₁ in a direction perpendicular to that of the split luminousfluxes. On the other hand, the main polarized beam component S_(f2) 'having the frequency f₂ is split into P_(f2) " and P_(f2) " incorresponding directions x and y. Furthermore, Δs_(f2) " and Δp_(f2) "exist in a mixed manner as noise components each having a frequency f₂in a direction perpendicular to that of the split luminous fluxes.

As, described above, the noise components ΔP_(f2) " and ΔS_(f2) " existin a mixed manner in the two luminous fluxes P_(f1) " and P_(f2) " whichhave passed through the polarization beam splitter 20. However, each ofthe noise components ΔS_(f1) " and ΔS_(f2) " has the same frequency asthat of the corresponding main polarized beam components P_(f1) " andP_(f2) ". Therefore, optical beats cannot take place in each of theluminous fluxes and such optical beat signals are not photo-electricallydetected as the reference signals via the lens 21, the referencediffraction grating 22 and the detector 23.

The reference optical beat signal obtained by the reference detector 23does not substantially include an error signal which degrades theaccuracy. Thus, a reliable and stable photo-electric signal can beobtained.

On the other hand, the two luminous fluxes S_(f1) " and S_(f2) "reflected by the polarization beam splitter 20 including the noisecomponents Δp_(f1) " and Δp_(f2) " in a mixed manner illuminate thediffraction grating mark WM on the wafer from two directions via therelay systems 24 and 25, the beam splitter 26, the parallel plane plate35 and the objective lens 36.

However, since the noise components ΔP_(f1) " and ΔP_(f2) " in theluminous fluxes have, similarly to the reference signals, the samefrequencies as those of the corresponding main polarized beam componentsS_(f1) " and S_(f2) ", no beat is generated in each of the luminousfluxes. Therefore, each of the photoelectrically detected signalsbecomes a reliable and stable signal including only the positionalinformation of the reticle and the wafer.

As described above, the luminous fluxes which have been individuallyfrequency-modulated by the AOMs or the like are guided in such a mannerthat they run symmetrically to each other with the optical axis of thealignment optical system disposed therebetween, without any intersectiontaking place again, so that the diffraction grating marks RM and WM areilluminated from two direction by the luminous fluxes. Therefore, nolight having different frequency which causes a detection error isincluded in the luminous fluxes, providing a significant improvement.

It might be considered feasible for not only a main polarized beamcomponent but also a noise component to become elliptically-polarizedlight by the optical members disposed in the alignment optical systemaccording to the present invention. However, the noise componentexisting by a slight quantity in each of the luminous fluxes accordingto this embodiment shown in FIG. 3 has the same frequency as that of themain polarized beam component. Therefore, even if noise componentNy(f₁), for example having the same frequency f₁ and in the samedirection as the main polarized component Pry(f₁) having the frequencyf₁ exists as shown in FIG. 4, the two luminous fluxes do not beat priorto being applied to the diffraction grating marks RM and WM formed onthe reticle and the wafer. Therefore, reliable and stable optical beatsignals can be obtained.

Now, the optical path adjustment function of the parallel plane plates16a and 16b will be described.

The parallel plane plates 16a and 16b act to maintain the state in whichthe luminous fluxes are split from each other in order to make the lighttransmission system include no error signal. Furthermore, they shift theoptical paths, through which the polarized light passes, in accordancewith their inclinations. Therefore, the two optical paths can bearranged in such a manner that they are made to run parallel to eachother and separated with the optical axis of the alignment opticalsystem, behind the parallel plane plate disposed therebetween.

The pupil surface Pu of the alignment optical system as shown in FIG. 5is positioned at the positions at which the polarization beam splitters18 and 20 are positioned. Beam spots BS₁ and BS₂ formed by the twoluminous fluxes supplied from the light transmission system are formedon the pupil surface Pu with the optical axis positioned therebetween. Apair of parallel plane plates 16a and 16b disposed above the pupil are,as shown in FIG. 5, arranged to be rotatable about x and y-axes. Thebeam spots BS₁ and BS₂ formed on the pupil surface Pu move iny-direction when the parallel plane plates 16a and 16b incline withrespect to x-axis, while the same move in x-direction when they inclinewith respect to y-axis. Therefore, the angle of intersection of the twoluminous fluxes illuminating the diffraction grating marks WM and RMformed on the wafer or the reticle can be adjusted.

An angle θ of intersection of the two luminous fluxes for illuminatingthe diffraction grating marks WM and RM is the sum of the incidentangles (α,β) of the two luminous fluxes. Therefore, assuming that thewavelength of the laser light source is λ, the pitch of the diffractiongrating is P, the order of the diffracted light generated due to theluminous flux for illuminating the diffraction grating at an incidentangle α is n₁ (n₁ >0) and the order of the diffracted light generateddue to the luminous flux for illuminating the diffraction grating at anincident angle β is n₂ (n₂ <0), the intersection angle θ can be definedas follows:

    θ=sin.sup.-1 (n.sub.1 λ/P)-sin.sup.-1 (n2 λ/P) (9)

Therefore, the beam spots BS₁ and BS₂ on the pupil surface Pu can beadjusted in x-direction by inclining the parallel plane plates 16a and16b about y-axis in accordance with the predetermined intersection angleθ. Therefore, a proper intersection angle in accordance with the pitchesof the diffraction grating marks WM and RM can be obtained merely bychanging the inclination angle of the parallel plane plates.

However, even if the intersection angle θ has been adjusted, an opticalbeat signal of a high contrast and including accurate positionalinformation cannot be obtained in the case where the direction(generation direction) of the arrangement of interference fringes IFdesignated by a continuous line of FIG. 6A and the direction (thedirection of the pitch) of the arrangement of the diffraction gratingmarks WM and RM designated by a short dashed line deviate from eachother in the plane of pupil surface Pu. Therefore, the direction of thearrangement of the interference fringes IF must be further adjusted bymoving the positions of the beam spots BS₁ and BS₂ formed on the pupilsurface Pu.

The moving interference fringes IF formed on the marks WM and RM arearranged perpendicularly to a straight line connecting the two beamspots BS₁ and BS₂ on the pupil surface Pu. Therefore, in order to makethe direction perpendicular to the straight line connecting the two beamspots BS₁ and BS₂ to be the arrangement direction of the gratings of thediffraction grating marks WM and RM with the predetermined intersectionangle maintained, it is necessary for the parallel plane plate 16a to beinclined about x and y-axes respectively. Thus, as shown in FIG. 6B, thebeam spot BS₁ is moved in x-direction by ΔX and in y-direction by ΔY soas to be positionally compensated to the position BS₁ '.

In the case where the direction of the arrangement of the interferencefringes IF and the directions of the arrangement of the marks WM and RMare slightly deviated from each other, the parallel plane plate 16a isonly required to be one-dimensionally inclined about x-axis so as tomove the beam spot BS₁ in y-direction by ΔY and compensate the positionto BS₁ " as shown in FIG. 6C.

As described above, the direction of arrangement of the interferencefringes IF and the direction of arrangement of the marks WM and RM canbe maded to coincide with each other with a predetermined intersectionangle maintained by two-dimensionally inclining at least one of theparallel plane plates.

In order to automatically achieve the above-described coincidence, it ispreferable that the inclinations of the parallel plane plates 16a and16b for coinciding the intersection angle and the inclinations of theparallel plane plates 16a and 16b for coinciding the direction ofarrangement of the interference fringe and the direction of thearrangement of the diffraction grating mark with each other be feed-backcontrolled.

Specifically, when, for example, the intersection angle is adjusted, theinclination of each of the parallel plane plates must be controlled asdescribed above so as to make the contrast of the optical beat signals,which are photo-electrically detected, the maximum level. When thedirection of the arrangement of the interference fringe and thedirection of the arrangement of the marks are adjusted, the inclinationof each of the parallel plane plates are necessary to be controlled asdescribed above so as to make the contrast of the optical beat signals,which are photo-electrically detected, the maximum level. As a result,the deviation between them can be compensated.

As described above, according to this embodiment, parallel plane plateswhose inclinations can be varied two-dimensionally are disposed in thealignment optical system so as to adjust the optical paths. A parallelplane plate may be disposed in at least one of the optical paths.Another structure may be employed in which a plurality of parallel planeplates whose inclinations can be varied one-dimensionally are disposedin the alignment optical system in each of the optical paths.

Referring to FIGS. 6B and 6C, although the two beam spots are positionedaway from the optical axis by a short distance, the optical beat signalsto be detected are not influenced from this.

In order to compensate the deviation between the direction of thearrangement of the interference fringes and the direction of thearrangement of the marks WM and RM, an image rotator may be disposed inthe alignment optical system.

Now, a modification to this embodiment will be described.

Referring to FIG. 1, the polarization beam splitter 18 is replaced by aprism having a light-transmitting plane and a light-reflecting plane anda half-wave plate 60 is disposed between the parallel plane plate 16band the prism. Luminous flux emitted from the laser light source 10 andsplit into two portions by the polarization beam splitter 13 ismodulated by AOMs 15a and 15b so as to have different frequencies.S-polarized light modulated by the AOM 15a to have the frequency f₁ isreflected by the reflecting plane of the prism 18 via the reflectingmirror 17. On the other hand, p-polarized light which has been modulatedby the AOM 15b so as to have the frequency f₂ is made s-polarized lightby the half-wave plate 60 and is thereby transmitted by the transmittingplane of the prism 18.

Each of s-polarized light beams having different frequencies which hasbeen reflected and transmitted by the prism 18 is transmitted by thehalf-wave plate 19. Since the polarization directions of the luminousfluxes have been aligned to each other, the light quantity of theluminous fluxes supplied, by the polarization beam splitter 20, to thereference signal detection system and to the light transmitting systemfor illuminating the reticle and the wafer can be adjusted. The lightquantity necessary as reference light for photo-electrical detectingperformed in the detector 23 can be reduced. Therefore, luminous fluxhaving a large light quantity can be supplied to the diffraction gratingmarks RM and WM by adjusting the degree of rotation of the half-waveplate 19.

Another structure may be employed in which the polarization beamsplitter 13 is replaced by a beam splitter and the partial reflectingplane of the prism 18 is formed by a metal film. In this case, thehalf-wave plate 60 can be omitted from the structure since the polarizedplanes of the luminous fluxes made incident upon the prism 18 have beenaligned. Since the luminous flux emitted from the laser source 10 isdivided into two portions by the beam splitter so as to be subjected tothe independent frequency-modulation by the AOMs 15a and 15b, theoptical beat signal which is to be photoelectrically detected is notdegraded even if polarization components having the same frequency aremixed in each of the luminous fluxes having the different frequencies.That is, since the state in which the luminous fluxes are separatedwithout the luminous flux components having different frequencies ismaintained, the luminous flux can illuminate the diffraction gratingmarks RM and WM from two directions without any beats.

Now, a second embodiment of the present invention will be described.

Referring to FIG. 8, a reticle 101 having a predetermined circuitpattern thereon is held by a reticle stage 102 which can movetwo-dimensionally. The pattern formed on the reticle 101 is uniformlyilluminated with exposing light supplied from an illuminating opticalsystem 130 so as to be imaged on the wafer 104 via a projecting lens103. A wafer mark WM of the alignment diffraction gratings is formed onthe wafer 104. The wafer 104 is held by a stage 105 which can movetwo-dimensionally.

In order to independently detect the x directional, y directional androtational (θ) positions on the reticle stage 102 and the wafer stage105, interferometers 145 and 147 are provided. The stages are moved ineach of the directions by motors 144 and 146.

Laser beams emitted from a laser light source 110 are expanded by a beamexpander 111 to a predetermined beam diameter and are divided into twoportions by a beam splitter 112. The luminous flux reflected by the beamsplitter 112 is made incident upon an acoustooptical modulator 114a(AOM). On the other hand, luminous flux transmitted by the beam splitter112 is made incident upon an AOM 114b via a reflecting mirror 113. Wheneach of the luminous fluxes is subjected to the frequency modulation bythe corresponding AOMs 114a and 114b, they become luminous fluxes havingdifferent frequencies. They are relayed by lenses 115a, 115b and 116 sothat luminous fluxes illuminate the field stop FS from two directions atpredetermined incident angles (±θ_(FS)).

As a result of the application of the alignment light, interferencefringes moving on the field stop FS are formed. Assuming that thewavelength of the alignment light is λ, the pitch of the interferencefringes becomes: ##EQU9##

Assuming that the luminous flux which has passed through the AOM 114a isfrequency-modulated into the frequency f₁ and the luminous flux whichhas passed through the AOM 114b is frequency-modulated into thefrequency f₂, the interference fringes move at a speed of P_(FS) ×|f₁-f₂ |.

The alignment light beams which have passed through the field stop FScause two coherent light beams having different frequencies from eachother to be applied to the diffraction grating mark WM formed on thewafer at incident angles (±θ_(WM)) which become symmetric with respectto the normal direction (the direction of the optical axis) via areflecting mirror 117, an objective lens 118, a beam splitter 119, adichroic mirror 120 and a projecting lens 103. As a result, interferencefringes moving on the wafer mark are formed similarly to those on theabove-described field stop FS. The pitch P_(IWM) of the interferencefringes becomes: ##EQU10## The interference fringes move at a speed ofP_(IWM) ×|f₁ -f₂ |.

At this time, ±n-order diffracted light is generated in the normaldirection of the wafer mark WM. The above-described incident angleθ_(WM) can be given assuming that the pitch of the diffraction gratingmark WM is P_(WM) and the wavelength of the alignment light is λ:##EQU11##

The ±1-order diffracted light generated in the normal direction isphoto-electrically detected by a detector 123 via the projecting lens103, the dichroic mirror 120, the beam splitter 119, the objective lens121 and the spatial filter 122, the detector being positioned inconjugation with the pupil of the projecting lens 103.

The spatial filter 122 is positioned in conjugation with the pupil ofthe projecting lens 103 and acts to pass diffracted light from the wafermark WM passing in the direction of the optical axis.

A phase detection system 141 detects the phase difference betweenoptical beat signal S_(WM) obtained by the detector 123 and the drivesignal difference |DS₁ -DS₂ | between the drive signals for the AOM 114aand 114b. In accordance with the phase difference, a control system 142controls the wafer stage 105 in such a manner that the phase differencebecomes zero. Thus, the position alignment of the wafer, that is, thealignment is conducted.

The above-described field stop FS restricts the illumination region onthe wafer diffraction grating mark and shields the detector from theincidence of a portion of the alignment light which has been reflectedor scattered by the wafer. However, if diffracted light is generated atthe edge of the field stop FS, the detection accuracy is degraded, aswill be described now.

As designated by continuous line shown in FIG. 9, coherent alignmentlight beams LB₁ and LB₂ having different frequencies from each otherilluminate the field stop FS from two directions at predeterminedincident angle θ_(FS). After the light beams LB₁ and LB₂ have passedthrough the lens 103a forming a portion of the projecting lens 103, twobeam spots BS₁ and BS₂ are formed on the pupil surface P (a Fourierplane) of the projecting lens. The alignment light which has passedthrough the lens 103b forming a portion of the projecting lens 103illuminates the illumination region of the diffraction grating mark WMformed on the wafer from two directions at predetermined incident angles(±θ_(WM)).

The incident angle (±θ_(WM)) is arranged so as to generate ±1-orderdiffracted light in the normal direction (in the direction of theoptical axis) of the wafer mark WM. Therefore, in order to conduct theideal alignment, only ±1-order diffracted light LB₁ (+1) and LB₂ (-1)generated in the normal direction by the wafer mark WM must be detected.However, when the field stop FS is illuminated with alignment lightbeams LB₁ and LB₂ having different frequencies from each other andhaving predetermined intersection angles (±θ_(FS)) from two directions,diffracted light is generated from the field stop FS. The diffractedlight degrading the alignment accuracy is analyzed as follows:

Since the two alignment light beams LB₁ and LB₂ made incident upon thefield stop FS can be treated as plane waves, Franformer analysis can beconducted on the pupil surface P of the projecting lens. Quantitatively,diffraction intensity distribution DF₁ due to the alignment light LB₁and the diffraction intensity distribution DF₂ due to the alignmentlight LB₂ are formed symmetrically with respect to the pupil center O onthe pupil surface P (on the Fourier plane) as shown in FIG. 10.Therefore, diffracted light of the alignment light LB₂ having intensityI_(N) is mixed into the alignment light LB₁ having intensity I_(O)forming the beam spot at the position BS₁. Diffracted light of thealignment light LB₁ having intensity I_(N) is mixed into the alignmentlight LB₂ having intensity I_(O) forming the beam spot at the positionBS₂.

Therefore, as designated by dashed line of FIG. 9, noise diffractedlight 1b₁ generated due to the alignment light LB₁ having the frequencyf₁ passes through the same optical path for the alignment light LB₂having the frequency f₂. The alignment light LB₂ and the noise light 1b₁beat and illuminate the wafer mark WM at an incident angel of -θ_(WM).

On the other hand, noise diffracted light 1b₂ generated due to thealignment light LB₂ having the frequency f₂ also passes through the sameoptical path for the alignment light LB₁ having the frequency f₁. Thealignment light LB₁ and the noise light 1b₂ beat and illuminate thewafer mark WM at an incident angel of +θ_(WM).

FIG. 11 illustrates a state in which diffracted light is generated onthe wafer mark WM. As shown in FIG. 11, -1-order diffracted light LB₂(-1) of the alignment light LB₂ and -1-order- diffracted light 1b₁ (-1)of the noise diffracted light 1b₁ are generated in the normal directionof the mark WM, and +1-order diffracted light LB₁ (+1) and +1-orderdiffracted light 1b₂ (+1) of noise diffracted light 1b₂ are generated.

As is shown from FIG. 10, the diffracted light of each of the alignmentlight beams has been mixed into center O of the pupil surface of theprojecting lens. Therefore, in the state shown in FIG. 9, the noisediffracted light 1b₁ ' and 1b₂ ' of each of the alignment light due tothe field stop FS passes on the optical axis and beats with each otherso that the wafer mark WM is vertically illuminated. As a result,0-order diffracted light 1b₁ '(0)' and 1b₂ '(0)' are generated in thenormal direction of the wafer mark WM.

As described above, the detector positioned in conjugation with thepupil surface P of the projecting lens detects all of light beamsgenerated in the normal direction of the wafer mark. Therefore, if noisediffracted light is included in the detected light, the optical beatsignal detected includes an excessive detection error.

The physical phenomenon will now be further analyzed.

Assuming that the amplitude of the +1-order diffracted light LB₁ (+1)generated in the normal direction of the wafer mark WM due to thealignment light LB₁ is A₁, the amplitude of the -1-order diffractedlight LB₂ (-1) generated in the normal direction of the wafer mark WMdue to the alignment light LB₂ is A₂, the amplitude of -1-orderdiffracted light 1b₁ (-1) generated in the normal direction of the wafermark WM due to the noise light diffracted light 1b₁ passing through thesame optical path for the alignment light LB₂ is b₁, and the amplitudeof +1-order diffracted light 1b₂ (+1) generated in the normal directionof the wafer mark WM due to the noise light diffracted light 1b₂ passingthrough the same optical path for the alignment light LB₁ is b₂, theoptical beat signal S_(WM) which is photoelectrically detected by thedetector can be expressed by the following equation:

    S.sub.WM =A.sub.1 cos (ω.sub.1 t+φ.sub.1)+A.sub.2 cos(ω.sub.2 t+φ.sub.2)+b.sub.1 cos(ω.sub.1 t+δ.sub.1)+b.sub.2 cos(ω.sub.2 t+δ.sub.2) (10)

Since the signal detected by the detector is the time average of theintensity (that is, the square of the amplitude), the intensity of thedetected signal when Δω=ω₁ -ω₂ becomes as follows: ##EQU12##

When, for example, the wafer is displaced by x in X direction lettingthe pitch of the wafer mark WM be P_(WM), the phase differencecorresponding to this becomes: ##EQU13## In order to make thedescription easier, it is provided that A₁ =A₂ .tbd.A, b₁ =b₂ .tbd.b andthat all of phases other than the phase difference due to theabove-described movement of the wafer are zero. Equation (11) can now beexpressed by the following equation: ##EQU14##

As is shown from Equation (12), the optical beat signal observed by thedetector becomes only the second and the third terms on the right sideand only the phase of the second term is changed due to the displacementof the wafer in direction X. Furthermore, the third term represents thenoise component which has no relationship with the displacement of thewafer in X direction. Therefore, it is apparent that the phasedifference between the optical beat signal S_(W) observed by thedetector and the drive signal DS for the AOM can be detected as thedifference from the synthesized waveform of the second the third termson the right side of Equation (12).

Furthermore, the phase change due to the displacement x of the wafer indirection x can be expressed by the following equation: ##EQU15##

Due to the bad influence of the error factor in the third term ofEquation (12), the phase difference which is actually detected becomesα_(p) as shown in FIG. 12, and the phase error becomes its maximum valuewhen α=π/2.

In this case, assuming that A² +b² >>2Ab, the phase error α_(e)(=α-α_(p)) can be approximated as ##EQU16##

Furthermore, assuming that A>>b, Equation (14) can be approximated as:##EQU17##

As described above, the alignment is conducted in such manner that therelative phase difference between the drive signal |DS₁ -DS₂ | of theAOM and the optical beat signal SWM is made to correspond to thedisplacement of the wafer. Therefore, the phase error acts as apositional error which excessively influences the detection accuracy.

Assuming that the phase error is α_(e) and the pitch of the wafer markof the diffraction grating formed and the wafer is P_(WM), thepositional error X_(e) of the wafer becomes as follows since the pitchof the interference fringes formed on the wafer mark is P_(WM) /2:##EQU18##

For example, assuming that the pitch of the wafer mark WM is 8 μm andthe positional error of the wafer is 0.02 μm or less, the followingequation can be obtained from Equation (13): ##EQU19##

In this case, the ratio of the noise diffracted light 1b₁ and 1b₂included in each of the alignment light LB₁ and LB₂ due to the fieldstop FS must meet the following condition since b/A=α_(e) /2 asexpressed in Equation (15): ##EQU20##

Then, the amplitude of the noise diffracted light actually mixed intoeach of the alignment light on the pupil surface P of the projectinglens is obtained. As described above, since the Franformer analysis canbe conducted on the pupil surface P, the sine value of the emissionangle of the diffraction light generated from the field stop FS can bearranged to be ξ which is the coordinate on the pupil surface.Therefore, the amplitude distribution of each of the diffracted light onthe pupil surface P can be given by the following equation: ##EQU21##(Wherein a combination of + and - signs between a denominator and anumerator is excluded) where the amplitude of the diffracted light is b,the amplitude of a plane wave incident upon the field stop is A0, thewavelength of the alignment light beams LB₁ and LB₂ is λ, the width ofthe field stop is w, the distances from center O of the pupil surface Pto the beam spots BS₁ and BS₂ are ±ξ₀ (=sin ±θ_(FS) =±λ/2P_(FS)), theincident angle of the alignment light beams LB₁ and LB₂ illuminating thefield stop FS are ±θ_(FS), the pitch of the interference fringes formedon the field stop is P_(FS) and sin0/0=1.

FIG. 13 is a graph of Equation (18) where a curve A1 shows thediffraction amplitude distribution due to the alignment light LB₂ on thepupil surface P of the projecting lens and curve A2 show the diffractionamplitude distribution due to the alignment light LB₂ on the pupilsurface P of the projecting lens. The two curves can be also explainedin the case on the pupil surface P of the projecting lens from thediffraction intensity distribution shown in FIG. 10.

For example, when the amplitude b of the noise diffraction light 1b₂ atthe beam spot position BS₁ (ξ=+ξ₀) of the alignment light LB₁ asdesignated by the curve A1 on the pupil surface P of the projecting lensis obtained, or when the amplitude b of the noise diffraction light 1b₁at the beam spot position BS₂ (ξ=-ξ₀) of the alignment light LB₂ asdesignated by the curve A2, is obtained, Equation (18) becomes asfollows: ##EQU22##

For example, when the pitch of the interference fringes formed on thefield stop is 4 μm and the shape of the field stop is in the form of arectangle having a width w=50 μm, Equation (19) becomes as follows:##EQU23##

Therefore, the above-described value is larger than b/A0≦0.016 obtainedin Equation (16). That is, it is difficult to secure an accuracy betterthan 0.02 μm or less in the structure in which the rectangular fieldstop FS is employed due to the influence of the diffraction from thefield stop FS.

The amplitude b of either the noise light passing through the center O(ξ=0) of the pupil surface P of the projecting lens after it has passedin the direction of the optical axis due to the field stop FS can beobtained from Equation (18) as follows: ##EQU24## Assuming that thewidth w of the field stop and the pitch P_(FS) of the interferencefringes formed on the field stop are determined to be theabove-described values, the following relationship can be obtained:##EQU25##

As a result, the value becomes larger than 0.016 obtained by Equation(16). Therefore, an accurate alignment cannot be conducted.

Accordingly, the shape of the edge of the field stop FS is designed asfollows in consideration of the direction of the groove of thediffraction grating mark on the wafer.

As shown in FIG. 14, edges e1 and e2 of the field stop FS extend indirection X corresponding to the direction of the arrangement of thewafer mark WM and edges e3 and e4 extend with inclined by θ° todirection Y (perpendicular to the direction of the arrangement)corresponding to the direction of the groove of the wafer mark WM.

As shown in FIG. 9, coherent light LB₁ and LB₂ having differentfrequencies from each other are applied to the field stop FS having aparallelogram shape from two directions at predetermined incident angles(±θ_(FS)) in such a manner that the coherent light beams LB₁ and LB₂become symmetrical with respect to the optical axis. As a result, thedirection of generation of the diffracted light is, as shown in FIG. 15,inclined by θ° with respect to direction ξ in accordance withinclination θ of the edge pair e3 and e4. The direction of generation ofthe diffracted light due to the other edge pair e1 and e2 becomes indirection η. The ξ-η coordinate on the pupil surface corresponds to thesine of the angle of emission of the diffracted light in direction XYgenerated from the field stop FS. Therefore, a diffraction intensitydistribution along a direction inclined by θ° with respect to directionξ centered about each of the beam spot positions BS₁ and BS₂ and anotherdiffraction intensity distribution along direction η are formed on thepupil surface P of the projecting lens.

The diffraction intensity of the alignment light LB₁ forming the beamspot BS₁ comprises diffraction intensity distribution DF₁ which damps inaccordance with the distance from the origin O as shown in FIG. 16,where a two dimensional expansion having external values of thediffraction intensity present at the positions designated by black dotsis shown. The pupil center O and the beam spot position BS₂ (-ξ0, 0)receive slight degradation from the noise diffracted light which istwo-dimensionally distributed.

Therefore, it is necessary for the shape of the opening of the fieldstop to be capable of reducing the noise light which can reach the lightreceiving surface of the detector disposed at a position in conjugationwith the pupil surface (or a position in conjugation with the wafer).That is, when the alignment light beam LB₁ and LB₂ are transmitted tothe wafer mark WM, the shape of the opening must to be able to reducethe influence of the diffracted light of the field stop FS upon thepupil center or the beam spot positions BS₁ and BS₂ of the alignmentlight on the ξ-η coordinate on the pupil surface P. Then, theinclination q of the edges e3 and e4 is determined as follows:

Analysis of the amplitude distribution on the coordinate ξ-η on thepupil surface shown in FIG. 16 gives ##EQU26## (wherein a combinationof + and - signs between a denominator and a numerator excluded) wherethe length of the longitudinal edge of the field stop FS extending inthe direction (X-direction) of the arrangement of the wafer mark WM isW, the length of the lateral edge perpendicular (Y-direction) to thelongitudinal edge is h, the inclination of the lateral edge is θ and sin0/0=1.

As shown in FIG. 15, the amplitude of the noise

light caused of mixture by diffracted light DF₂ due to the alignmentlight LB₂ into the alignment light LB₁ passing through the beam spotposition BS₁ at coordinate (+ξ0, 0), or the amplitude of the noise lightcaused of mixture by diffracted light DF₁ due to the alignment light LB₁into the alignment light LB₂ passing through the beam spot position BS₂at coordinate (-ξ0, 0) can be obtained from Equation (20) as follows:##EQU27## where the sine of angle θ_(FS) made by light which reaches thecenter O of the pupil and the light which reaches the beam spot BS₁ (orBS₂), that is sine θ_(FSq) is ξ0, the wavelength of the alignment lightLB₁ and LB₂ is λ, the pitch of the interference fringes formed on thefield stop is P_(FS) and ξ0=λ/2 P_(FS).

Assuming that the lens 118 and the projecting lens 103 meet the sinecondition as shown in FIG. 17, the synthetic magnification of the lensdisposed between the field stop FS and the wafer becomes as follows:##EQU28##

Assuming that the pitch of the wafer mark WM is P_(WM), the incidentangle of each of the alignment light beam illuminating the diffractiongrating mark WM on the wafer mark is θw and sin θw=ξ0', detection of±n-order diffracted light from the wafer mark WM due to the alignmentlight LB₁ and LB₂ from Equation (22) gives the following relationship:

    ξ0=Mξ0'=nMλ/P.sub.WM

    P.sub.WM =2 nMP.sub.FS

Therefore, Equation (21) can be expressed as follows: ##EQU29##

As is shown from Equations (21) and (23), the factors influencing thealignment accuracy can be removed by determining the most suitable rangeof the edge inclination of the opening formed in the field stop FS.

Therefore, in order to make the numerator of Equation (23), the maximumvalue in consideration of the case in which the error included in thediffracted light becomes maximum, the value of the sine term is made 1.As a result, the following relationship is held from Equation (21):##EQU30##

From Equation (23), the following equation can be obtained: ##EQU31##

For example, in order to secure the alignment accuracy better than 0.02μm on the wafer, it is assumed that b/A0≦0.016, the syntheticmagnification of the lens disposed between the field stop FS and thewafer is 1 (θw=θ_(FS)), the detected diffracted light is ±1-orderdiffracted light, the pitch PWM (=2P_(FS)) of the diffraction gratingmark WM on the wafer is 8 μm, the length W of the longitudinal edge ofthe field stop FS is 50 μm, and the length h of the lateral edgeperpendicular to the longitudinal edge is 30 μm. As a result,inclination θ of the lateral edge becomes θ≧3.9° from Equations (24) and(25).

Therefore, in order to reduce the influence of noise diffracted light tobe mixed into the alignment light LB₁ and LB₂ the edges e3 and e4 of theopening must be arranged so as to meet θ≧3.9°.

In general, a satisfactory alignment accuracy can be secured byproviding a field stop FS having an edge inclination θ with which thevalue of the right side of Equation (24) or (25) exceeds the value ofthe left side of the same.

Assuming that A is A0, X_(WM) is allowable alignment error (μm) on thewafer and Xe of Equation (16) is X_(WM), the following equation can beobtained: ##EQU32##

Therefore, the following equations can be deduced from Equations (24) to(26): ##EQU33##

In the case where the field stop having the opening which meetsEquations (27) and (28) is provided in an alignment optical system, theinfluence of the diffracted light 1b₁ ' and 1b₂ ' generated in thedirection of the optical axis and due to the field stop FS is not takeninto consideration. Therefore, it is preferable that, for example, alight shield member 150 which allows only 0-order light to pass throughas shown by a dashed line of FIG. 8 be provided in an optical path at aposition more adjacent to the wafer than the position of the field stopFS in the alignment optical system. As a result, the alignment accuracybetter than X_(WM) (μm) on the wafer can be assured.

Then, in order to reduce the influence of the diffracted light 1b₁ ' and1b₂ ', generated in the direction of the optical axis due to the fieldstop, the influence of the noise diffracted light which enters theoptical path on the optical axis is obtained by arranging ξ=0 and η=0 inEquation (20). Similarly to Equations (24) and (25), making thenumerator of Equation (20) the maximum value, that is making the valueof sine term 1 gives the following equations: ##EQU34## where A0represents the amplitude of either of the alignment light illuminatingthe field stop FS from two directions and b represents the amplitude ofthe noise diffracted light generated in the direction of the opticalaxis due to the field stop FS.

In order to eliminate the noise light, it is arranged such that A ofEquation (15) is A0, Xe of Equation (16) is X_(WM). As a result,Equations (29) and (30) can be expressed as follows from theabove-described two equations: ##EQU35##

Therefore, in general, the influence of the diffracted light 1b₁ ' and1b₂ ', generated in the direction of the optical axis due to the fieldstop FS can be reduced by constituting the structure in such a mannerthat the edges e3 and e4 of the opening of the field stop FS arearranged to meet Equation (31) or (32). Furthermore, the noisediffracted light which can be mixed into the alignment light can besimultaneously reduced.

That is, when the field stop FS is arranged to meet Equation (31) or(32), all of the bad influence of noise diffracted light influencing onthe optical beat signal to be detected can be eliminated.

For example, assuming that M=1 (θ_(WM) =θ_(FS)), P_(WM) (=2P_(FS))=8 μm,w=50 μm, h=30 μm and X_(WM) ≦0.02 μm, a becomes θ≧16.7° from Equation(31) or (32). Therefore, the structure is necessary to meet thiscondition is thus obtained.

In the case where the edges of the field stop FS are arranged to meetEquation (31) or (32), the light shield member 150 shown in FIG. 8 canbe omitted from the structure.

Now, a third embodiment of the present invention will be described.

In the case where the field stop is arranged in the form of arectangular shape, the third term 2Ab cos ωt of Equation (12) directlyinfluences the detected signal. The value of Ab can be expressed asfollows from Equation (18): ##EQU36##

FIG. 18 is a graph which illustrates Equation (33). As shown in thisdrawing, there is no relationship with the movement of the wafer and theluminous flux which has passed through the position ±ξ on the pupilsurface P of the projecting lens becomes the optical beat signal causingthe maximum error.

When +1-order diffracted light is detected from the wafer mark WMassuming that the synthetic magnification of the lens disposed betweenthe field stop FS and the wafer is M (=sin θ_(FS) /sin θ_(WM)) and thewavelength of the alignment light is λ, the pitch P_(FS) of theinterference fringes formed on the field stop FS becomes: ##EQU37##where sin θ_(WM) =λ/P_(WM).

As shown in FIG. 19, assuming that the length of the longitudinal edgecorresponding to the direction of the arrangement of the wafer mark WMon the field stop FS having a rectangular opening is W and symbol mrepresents an integer, when the structure is arranged to meet W=mP_(FS)(=mP_(WM) /2M), Equation (33) becomes: ##EQU38##

When the value of Ab at the position +ξ0 (=Mλ/P_(WM)) is obtained fromEquation (35), it can be obtained as follows: ##EQU39## As shown in FIG.20 which is a graph showing Equation (35), the positive sign and thenegative sign of the value of Ab are inverted in the vicinity of ±ξ0 onthe Fourier plane. Therefore, the influence of the noise light on thedetector can be reduced. Although the ±1-order diffracted light from thewafer mark WM is detected according to the above-described embodiments,the present invention may, of course, be effectively applied to a casein which ±n-order diffracted light from the wafer mark WM is detected.

According to this embodiment, the structure is so arranged that only theinfluence of the noise light which can be mixed into the alignment lightLB₁ and LB₂ for illuminating the diffraction grating mark WM on thewafer from two directions at predetermined incident angles is intendedto be reduced. Therefore, it is preferable as described earlierregarding FIG. 8, that a light shield member 150 which allows onlyalignment light LB₁ and LB₂ having the different frequencies from eachother to pass through be disposed in an optical path closer to the waferthan the position of the field stop FS in the alignment optical system.

According to the above-described first to third embodiments, the opticalbeat signal including the positional information is detected from thewafer and the alignment is thus conducted in such a manner that thephase difference from a signal for driving the acoustooptical modulator(AOM) is made zero. Another structure may be employed in which referencelight obtained by splitting alignment light beams LB₁ and LB₂ which havebeen frequency-modulated by the acoustooptical modulator by a beamsplitter is photo-electrically detected and the alignment is conductedin such a manner that the phase difference between the above-describedreference signal and the optical beat signal including the positionalinformation from the wafer is made zero.

Now, a fourth embodiment of the present invention will be described withreference to FIG. 21. Referring to FIG. 21, the same elements as thoseshown in FIG. 8 are given the same reference numerals.

According to this embodiment, alignment light beams LB₁ and LB₂ areapplied to the wafer via the reticle and the projecting lens so that anoptical beat signal including the positional information from thereticle and an optical beat signal including the positional informationfrom the wafer are photo-electrically detected so that the positionalalignment between the reticle and the wafer is conducted.

The opening in the field stop FS in the alignment optical system is, asshown in FIG. 14, in the form of a parallelogram. The alignment lightbeams LB₁ and LB₂ which have passed through the field stop FS illuminatethe diffraction grating mark RM on the reticle from two direction atpredetermined incidental angles (±θ_(WM))

The relationship between the incident angles (±θ_(FS)) when thealignment light LB₁ and LB₂ illuminates the field stop FS and theincident angle (±θ_(WM)) when the same illuminates the reticle mark RMbecomes as follows assuming that the imaging magnification of the lens118 disposed between the field stop FS and the reticle is m₁ similar tothe case shown in FIG. 17:

    ±θ.sub.FS =±m.sub.1 θ.sub.RM

As shown in FIG. 22A, a light transmissible opening P0 of aparallelogram having edges inclined by θ° is positioned neighboring tothe reticle mark RM on the reticle similarly to the edges e3 and e4 ofthe above-described field stop FS. The alignment light illuminates theregion which simultaneously covers the reticle mark RM and the openingP0.

The ±1-order diffracted light generated in the normal direction due tothe alignment light beams LB₁ and LB₂ which have illuminated the reticlemark RM from two directions reaches a spatial filter 201 via thedichroic mirror 120, the beam splitter 119 and a lens 200.

The spatial filter is positioned in conjugation with the pupil of theprojecting lens 103 and acts to allow only luminous fluxes passing inthe direction of the optical axis to pass (only the ±1-order diffractedlight from the reticle mark RM and the ±1-order diffracted light fromthe wafer mark WM to be described later). Thus, unnecessary luminousfluxes can be filtered.

The ±1-order diffracted light from the reticle mark RM passes throughthe spatial filter 201 and is reflected by a beam splitter 203. Then, itreaches a detector 205 via a mask member 204 at which an optical beatsignal S_(RM) including the positional information of the reticle isphoto-electrically detected.

The mask member 204 is positioned in conjugation with the reticle and asshown in FIG. 23A, an opening A_(RM) corresponding to the reticle markis formed so that only ±1-order diffracted light from the reticle markRM is allowed to pass through.

On the other hand, the alignment light beams LB₁ and LB₂ which havepassed through the opening P0 in the parallelogram shape shown in FIG.22A illuminates the diffraction grating mark WM on the wafer via theprojecting lens 103 at a predetermined incident angle θ_(WM) from twodirections. The wafer mark WM is, as shown in FIG. 22B, positioned at aposition corresponding to the reticle opening P0.

The relationship between the incident angles (±θ_(RM)) when thealignment light LB₁ and LB₂ illuminates the reticle mark RM and theincident angle (±θ_(WM)) when the same illuminates the wafer mark WMbecomes as follows assuming that the imaging magnification of the lens118 disposed between the field stop FS and the reticle is m₂ similarlyto the case shown in FIG. 17:

    ±θ.sub.RM =±m.sub.2 θ.sub.WM

The pitch of the reticle mark RM and that of the wafer mark WM hold thefollowing relationship:

    P.sub.WM =m.sub.2 P.sub.RM

The ±1-order diffracted light generated in the normal direction of thewafer mark WM by the alignment light LB₁ and LB₂ illuminating the wafermark with the above-described relationship about the incident anglesagain passes through the parallelogram opening P0. At this time, also0-order diffracted light of the alignment light LB₁ and LB₂ which isregularly reflected and having the same ejection angle as the incidentangle by the wafer mark is made incident upon the transmission openingP0 at an incident angle of ±θ_(RM), and is again diffracted. Therefore,it can be considered that the ±1-order diffracted light receives a badinfluence. Accordingly, the inclination q of the edge of the opening P0according to this embodiment is determined so as to meet Equation (31)or (32) deduced in accordance with the above-described embodiments.

The ±1-order diffracted light which has passed through the parallelogramopening P0 reaches the detector 207 via the dichroic mirror 120, thebeam splitter 119, the lens 200, the spatial filter 201, the lens 202,the beam splitter 203 and the mask member 206.

The mask member 206 is disposed at a conjugational position with thewafer and has an opening A_(WM) corresponding to the wafer mark WM asshown in FIG. 23B so that only ±1-order diffracted light from the wafermark is allowed to pass through.

As described above, the mixture of the noise diffracted light into theoptical beat signals from the marks RM and WM is reduced by means of thefield stop FS and the reticle opening P0, and the optical beat signalsare independently detected by the detectors 205 and 207. Therefore, areliable optical beat signal can be obtained, causing an accuratealignment to be obtained.

The shape of the field stop FS and that of the reticle opening P0 arenot limited to the parallelogram. For example, a rhomboid or a trapezoidshape can be employed.

In the case where the noise diffracted light is mixed into the detectedlight, a mask member having an opening of the above-described shape oran opening having edges whose length is an intergral multiple of thepitch of the interference fringes may be positioned in conjugation withthe wafer or the reticle in the detection system.

According to the above-described embodiments, a pair of alignment lightbeams are applied to both the reticle mark RM and the wafer mark WM frompredetermined two directions and the optical beat signal of ±n-orderdiffracted light generated in parallel with the optical axis is detectedby the detection system. The condition for generating the ±n-orderdiffracted light in the same direction is defined by the relationshipbetween the incident angle of the pair of the alignment light beams andthe pitch of the diffraction grating. According to the above-describedembodiments, the alignment can be conducted even if the incident angleof the alignment light is not changed and only the pitch P_(RM) of thediffraction grating forming the reticle mark is halved. That is,assuming that the pitch of the reticle mark RM is P_(RM) ', thefollowing relationship is held: ##EQU40##

Therefore, the diffraction angle of the ±n-order diffracted light isdoubled in accordance with the halving the pitch of the reticle mark.

FIG. 24 illustrates the above-described state. When the alignment lightLB₁ illuminates the reticle mark RM at an incident angle θ_(RM),-1-order diffracted light LB₁ (-1) passing reversely through the opticalpath for the alignment light LB₁, 0-order diffracted light LB₁ (0). indirection +2 θ_(RM) with respect to the direction of the passing of thediffracted light LB₁ (-1), +1-order diffracted light LB₁ (+1) indirection +4θ_(RM), and -2-order diffracted light LB₁ (-2) in direction-2θ_(RM) are respectively generated.

On the other hand, when the alignment light LB₂ illuminates the reticlemark RM at an incident angle θ_(RM), +1-order diffracted light LB₂ (+1)passing reversely through the optical path for the alignment light LB₂,0-order diffracted light LB₂ (0) in direction -2 θ_(RM) with respect tothe direction of the passing of the diffracted light LB₂ (+1), -1-orderdiffracted light LB₂ (-1) in direction -4 θ_(RM), and -2-orderdiffracted light LB₂ (+2) in direction +2 θ_(RM) are respectivelygenerated, where the diffracted light generated clockwise with respectto the 0-order light of the alignment light is arranged to be positivelight, and that generated counterclockwise is negative light.

Therefore, diffracted light beams which have been diffracted by thereticle mark and pass in the same direction interfere with each otherand each of the interference light is photo-electrically detected sothat an optical beat signal including the positional information of thereticle can be obtained. Therefore, a beam splitter or the like ispositioned in, for example, an optical path between the field stop FSand the reticle so as to separate LB₂ (+1) and the beat light due to LB₁(0) into independent optical paths and a detector is positioned at aposition in pupil-conjugation with the reticle. As a result, the beatlight including the positional information of the reticle can beextremely accurately photo-electrically detected.

Although the heterodyne coherent type position detection apparatus hasbeen described according to the above-described embodiments, the sameeffect can be expected from the homodyne coherent position detectionapparatus. That is, a pair of alignment light beams having the samewavelength are applied to the diffraction grating mark from twodirections so as to generate stationary interference fringes, and thealignment light and the diffraction grating mark are relatively moved sothat the detection signal similar to that obtained from the heterodynecoherent position detection apparatus can be obtained.

Although the invention has been described in its preferred form with acertain degree of particularly, it is understood that the presentdisclosure of the preferred form may be changed in the details ofconstruction and the combination and arrangement of parts to withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

We claim:
 1. A position detection apparatus comprising:a substratehaving a diffraction grating; means for supplying illuminating flux;means for splitting said illuminating flux into two luminous fluxes;means for modulating the frequency of at least one of said luminousfluxes by a predetermined value in order to obtain two luminous fluxeshaving frequencies that are different from each other; means for causingthe last-mentioned luminous fluxes to impinge upon said substrate insuch a manner that they pass along independent optical paths and thenare incident on said diffraction grating from different directions;means for adjusting an intersection angle made by said two luminousfluxes incident on said diffraction grating; means for adjusting thedirection of arrangement of interference fringes generated on saiddiffraction grating by said two incident luminous fluxes, thelast-mentioned adjusting means being disposed in at least one of saidoptical paths and including optical means which can be displaced forshifting a luminous flux received thereby in two directions orthogonalto each other; and means for photo-electrically detecting an opticalbeat signal due to said interference fringes.
 2. A position detectionapparatus according to claim 1, wherein said optical means includes aparallel plane plate whose inclination angle can be varied about twoaxes orthogonal to each other.